I've thought about this too.
I think that first off, any ex will have to disguise himself in dub-gear. You know, magazine bag, crimplene shirt, polyester tie from Tesco. Some Sven-type glasses would add an NHK touch.
The badge might be difficult, maybe someone could get hold of the latest one from the US, scan and post it for copying, print it and wear it, and claim to be from overseas.
Then, each infiltrator should have a quantity of copied genuine material that shows up the dubs for what they are. Say a bit from the Flock elders manual, a copy of the UN list that includes the WT, maybe some BOE letters.
Our secret agents would then sit here and there in the stadium, then move on, each time leaving a copy of this material on the seat that they have just vacated. Provided nothing is added to the original, many of these papers would be read by persons sat nearby.
Now of course they would scream apostasy plot, but nonetheless, much of the material would sit in the persons mind, so that they would recall it as time showed it was all true.
Englishman.